Variably timed intake and exhaust valves are used in a variety of engines, notably compression ignition internal combustion engines. Conventional engine systems utilize a rotating cam configured to control opening and/or closing of engine valves at desired times during an engine cycle. Variable valves provide some flexibility in opening or closing timing in such engine systems via actuators configured to control valve position separately from cam rotation. By varying the opening or closing timing of engine valves, operating strategies are enabled which can provide enhanced control over cylinder conditions during an engine cycle, enabling operating strategies to reduce emissions or facilitate engine braking, for example. As is well known in the art, however, these systems do not always consistently perform precisely as desired, and the performance characteristics of variable valves can vary under certain conditions.
One known application for variable valves relates to selectively holding open intake valves. In a typical engine cycle, a rotating cam opens an intake valve during an intake stroke in a particular cylinder, then permits the valve to return to a closed position approximately when the corresponding piston reaches a bottom dead center position and begins a compression phase. Variable intake valve actuation may be used to keep a particular intake valve in an open position past the point at which cam rotation would ordinarily allow the intake valve to close.
It is common for variable valves to be hydraulically actuated, via an intake valve actuator separate from the engine cam. In such a design, as an intake valve moves toward an open position under the influence of a rotating cam, hydraulic fluid is permitted to flow into a control cavity. A separate control valve may be then used to block fluid draining from the cavity, such that the intake valve actuator is hydraulically locked and inhibits closing of the intake valve. The intake valve may be held in an open position until such time as the control valve is adjusted to permit draining of fluid and consequent returning of the intake valve to a closed position. As alluded to above, however, certain performance characteristics such as the specific timing of variably actuated valve closing can depart from an optimal timing. Variations in timing can degrade overall engine performance, and emissions quality, and can render certain operating schemes unachievable.
One method of diagnosing problems in a variable valve engine relies upon the use of a pressure sensor coupled with a common rail that supplies actuation fluid to the intake valve actuator. Operation of variable valve actuators in an engine has been shown to be associated with certain changes in pressure in a common rail. Thus, an electronic controller may be coupled with the pressure sensor and configured to log changes in common rail pressure which correspond with expected changes from variable valve actuator operation. While such systems have performed relatively well over the years, they add expense and complexity to an engine system, and can suffer from reliability and warranty issues.
Another approach to monitoring certain aspects of engine performance and operation is known from United States Patent Application Publication No. 2005/0257604 to Zavarehi et. al. (“Zavarehi”). Zavarehi is directed to a method and system for determining engine cylinder power level deviations via an analysis of crankshaft speed fluctuations. To detect these power level deviations, the engine is operated at a steady state low idle condition, and engine speed data collected over a plurality of engine cycles. An averaged, filtered data set is compared to expected engine speed data. Substantial deviations from the expected speed data may indicate power level deviation in a particular cylinder. While Zavarehi provides an approach suitable for evaluating certain aspects of engine operation, other strategies concerned with variable valve performance are desirable.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.